Narrow streets are the safest

Twenty-four foot wide streets are the safest, according to a study by Swift and Associates and the City of Longmont, Colorado. The study, Residential Street Typology and Injury Accident Frequency, looked at 20,000 automobile accident reports over an eight year period in Longmont, and examined fire department records. This data was correlated by street and 13 variables.

“The most significant causal relationships to injury and accident were found to be street width and street curvature,” according to the report. “The analysis illustrates that as street width widens, accidents per mile per year increases exponentially, and that the safest residential street width is24 feet (measured from curb face).”

Peter Swift, principal of the Longmont town planning and engineering firm, says a literature search yielded no previous studies correlating street width and other characteristics to general public safety. “This study is not definitive,” the authors conclude. “Additional research must be accomplished to build the body of knowledge related to such an important safety issue.”

In addition to Swift, authors are Dan Painter, transportation planner for the City of Longmont, and Matthew Goldstein, intern for Swift and Associates. The study, currently in draft form, is undergoing peer review and has not been published. One of the more remarkable findings is relative importance of automotive to fire injuries.

One of the most common difficulties of gaining approval for narrow streets is objections from fire officials, who often predict dire consequences. There was one serious fire and a number of smaller fires during the study period. No injuries were reported. The serious fire was in an older section of the city with 28-foot street width and rear alley. There was no access problem, nor were there any other fire truck access problems reported during the eight-year period.

The report therefore found no increased fire injury risk due to narrow streets. By contrast, there were 227 automotive accidents resulting in injuries. According to the curve plotted by the study, 0.32 automotive injury accidents can be anticipated per year per mile on a 24-foot-wide street, compared to 1.21 on a 36-foot-wide street.

Even if narrow streets did create a moderately greater fire injury risk, still they would be safer than wide streets — because the risk of automotive injuries is so much greater than fire injuries. The study looked only at streets defined by the city as local (less than 2,500 average daily trips, or ADT). Arterial streets were excluded. ADT was estimated for each local street, but, interestingly, was not found to be a major factor in the accident rate of a street. Some of the most dangerous streets turned out to be wide thoroughfares, 36 feet to 44 feet, with relatively light traffic (less than 500 ADT).

A future study would do well to correlate ADT more closely to street widths and accident rates. On the narrowest streets (20 and 22 foot), the report found a slight rise in injury accidents, compared to the 24-foot street. However, many of these very narrow streets in Longmont result from anomalies, such as “half streets,” where funding was not available to complete an entire street. Such anomalies may result in unsafe conditions independent of street width. These streets, nevertheless, had fairly low actual numbers of injury accidents (about 0.25/mile/year), and the study results are therefore somewhat inconclusive concerning very narrow streets.